(a) Technical Field
The present disclosure relates generally to an engine oil composition. More specifically, it relates to a diesel engine oil composition with improved fuel efficiency and durability.
(b) Background Art
The fuel efficiency of an engine oil can be improved when drag torque and friction in the sliding region of the engine are reduced. When the viscosity of the engine oil is reduced, the drag torque may be reduced. However, the friction and wear in the sliding region of the engine may be increased in such a case. Therefore, to improve the fuel efficiency of an engine oil by reducing its viscosity, it seems necessary to use an additive or additives to maintain its viscosity at high temperatures, and to reduce the friction and wear of an engine.
In general, olefin copolymer is added to the engine oil as a viscosity index improver, and zinc alkyldithiophosphate and molybdenum are added together as an anti-wear agent.
The durability of the engine oil is affected by many factors, for example, a reduction in performance of the engine oil due to oxidation, the generation of sludge due to friction and wear, and the deterioration of the engine oil itself When the engine oil has been used under high and low temperatures for a long time, its durability is reduced due to the generation of sludge because of oxidation, thermal decomposition, thermal polymerization, and other reactions that have occurred. As such, an appropriate antioxidant is required to improve the oxidative stability of the engine oil.
When the friction and wear occur at high temperatures and in the boundary lubrication region, excessive heat is generated, which leads to the increase in the viscosity of the engine oil and the total amount of acid contained thereof. The generation of sludge is also increased, thereby reducing the durability of the engine oil. In such circumstances, an anti-wear agent is also needed to prevent the friction and wear. Further, when the engine oil is used under severe conditions, the viscosity index improver that may have been added to the engine oil will be destroyed, thereby causing the engine oil film to become thinner, which results in an increase in the friction and wear. In other circumstances, the friction and wear may be further increased when the anti-wear agent is depleted due to a long-term use.
In the situations when a diesel engine is employed, incomplete combustion products and soot particles, will be generated through the operation. As time goes on, the size of soot particles as generated increases due to attractive forces between molecules. It follows that the viscosity of the engine oil increases, which in turn increases the wear and the sludge generation. The generated soot and sludge may block the oil flow, which reduces the fuel efficiency and the durability of the engine oil. In circumstances like this, it is necessary to use a dispersant to disperse the soot particles. Furthermore, the carbon and sulfur contained in the fuel and engine oil could be oxidized during fuel combustion to produce sulfuric acid and nitric acid. As a result, the total acid amount increases in the engine oil. And polar organic compounds as generated will be accumulated in engine pistons. In such circumstances, it is also necessary to use a detergent to neutralize the acid and to prevent the accumulation of contaminants in the pistons.
U.S. Pat. No. 5,863,873 discloses an additive, which contain polymethylacrylate, zinc alkyldithiophosphate, and molybdenum dithiocarbamate. Korean Patent Publication No. 10-1999-0014470 discloses an engine oil containing molybdenum dithiocarbamate. However, according to these techniques, whenever the low-temperature viscosity of the engine oil is reduced, the high-temperature viscosity is also reduced, which is disadvantageous as the durability of the engine oil at high temperatures is reduced, and the wear resistance of engine parts is also reduced. Moreover, although these techniques provide the friction reduction effect in the boundary lubrication region, only insignificant friction reduction effect in the fluid lubrication region is observed.
Therefore, there is an urgent need in the field for engine oil compositions with improved fuel efficiency and durability.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.